Replace logind CanGraphical with udev monitoring

[emersion]

See https://lists.freedesktop.org/archives/systemd-devel/2020-April/044245.html

[ascent12]

One way to do this is to make wlr_session a type that the user is expected to deal with during initialization.

1. User creates a wlr_session
2. Listens for a "device added" signal
3. Creates a DRM backend from the device, possibly mulitple
4. Starts the backend

This would also help out with DRM device hotplugging, but is quite a breaking change. It doesn't fit the X11/Wayland backend initialization either, because they have no need for wlr_session. We could make the user just deal with the DRM backend acting differently, or add some kind no-op wlr_session that X11/Wayland would use.

Another easier but crappier way is to just block inside the wlr_session, waiting for a device to show up.

[emersion]

I don't know whether multiple DRM backends is a good idea. Could just have one DRM backend for all DRM devices.

[ascent12]

We still need a way to notify the user that a new device has appeared or disappeared. They may want to selectively enable/disable devices based on user configuration.

[emersion]

That's a good point. They also may want not to use udev and have a custom list of DRM devices to open.